Ever wondered why the ocean is so blue? Well, it isn’t reflection of the colour of the atmosphere that’s behind it. No, that popular explanation is wrong, though not completely. It’s just that water isn’t completely transparent. If you get enough water molecules together in one place, like in an ocean, then you can see the effect: Light that isn’t blue is absorbed, and light that’s blue isn’t.
Then, of course, water’s black sometimes. But that’s a different story.